1. Field of the invention
The present invention relates to apparatuses and methods for controlling the resistivity of ultra pure water to be used as cleaning water in the production of semiconductor or liquid crystal devices.
2. Background art
It is widely known that when a photomask substrate, silicone wafer, or glass plate is cleaned, or when a wafer is cut by a dicing machine, by using ultra pure water (resistivity≧18 M Ω−cm) in the process of producing semiconductor or liquid crystal devices, due to the high resistivity of the ultra pure water, static electricity is generated which causes dielectric 15 breakdown or adsorption of particulates, thereby exerting marked undesirable effects on the production rate of the substrate.
Accordingly, in order to avert such undesirable effects, a method has been generally known in which the resistivity of the ultra pure water is reduced to about 0.01-15 M Ω−cm by dissolving carbon dioxide in the ultra pure water.
Also, since the flow rate of the ultra pure water is largely changed in the processes of cleaning, dicing, etc. of a silicone wafer, it is required that the resistivity of the ultra pure water be unchanged even if the flow rate of the ultra pure water is vigorously changed. In order to make improvements with this regard, U.S. Pat. No. 6,158,721 discloses a method in which ultra pure water, which is supplied corresponding to the rate of consumption, is divided into two flows with constant proportions, one of which is larger than the other, by a distributing device. The smaller flow is supplied to a gas permeation membrane module so that carbon dioxide or ammonia is dissolved in the flow so as to be a certain concentration which is more than 90% of the equilibrium concentration determined by the gas pressure and the temperature of the ultra pure water. Then, the smaller flow of the ultra pure water, to which carbon dioxide or ammonia has been added is combined with the larger flow of untreated ultra pure water, and is mixed uniformly so that the resistivity of the combined ultra pure water may be constantly maintained even if the flow rate of the raw ultra pure water is changed.
However, in the above-mentioned method, since the mixing ratio of the ultra pure water, to which carbon dioxide or ammonia is dissolved at a high concentration in order to achieve a desired resistivity, with respect to the untreated raw ultra pure water, i.e., [ultra pure water to which carbon dioxide or ammonia is dissolved at high concentration]/[untreated raw ultra pure water], is very small, it is not easy to constantly maintain the above-mentioned mixing ratio when the flow rate of the ultra pure water is changed. Thus, according to the above method, it is sometimes unavoidable that the change in the resistivity of the ultra pure water increases.